The measurement apparatus for measuring spectral characteristics of the inside of a biological tissue is used for determining formation of new blood vessels or oxygen metabolism of hemoglobin attending on growth of tumor based on light absorption characteristics of a specific substance such as hemoglobin contained in blood, to thereby utilize the results for diagnosis.
Such an apparatus uses a near infrared beam having a wavelength of approximately 600 to 1,500 nm with excellent transmittance property for a biological tissue.
As a method of measuring spectral characteristics of the inside of a biological tissue, there is known a method utilizing a photoacoustic effect. An apparatus using this method irradiates the interior of a biological tissue with a pulse beam so that spectral characteristics of a local region can be measured from a photoacoustic signal that is generated based on light energy.
Intensity of the light applied to the interior of the biological tissue is attenuated by absorption and dispersion during the process of propagating in the biological tissue, and thus little light reaches a deep part of the tissue.
Conventionally, in order to solve those problems, there is proposed an apparatus in which two illumination optical systems are disposed at positions that are opposed to each other with respect to an inspection object, and the inspection object is illuminated from both sides thereof so that increased light can reach the deep part (see U.S. Patent application No. 2004/0127783).
In addition, there is proposed an apparatus in which optical fibers for irradiating the biological tissue with light and piezoelectric elements for detecting the photoacoustic signal are arranged alternately, or transparent piezoelectric elements through which light for irradiation can pass are used, whereby a detector for the photoacoustic signal is disposed on the same side as that of the illumination optical system (see Japanese Patent Application Laid-Open No. 2005-021380).
Further, there is proposed an apparatus in which a transducer for detecting a photoacoustic signal is disposed on the same side as that of the optical fiber for irradiating a biological tissue with light, and these are scan-driven along the surface of an inspection object (see U.S. Pat. No. 5,840,023).
Further, there is proposed an apparatus in which an inspection object such as a breast is pressed to be flat, and a plane for irradiating the flat inspection object with light is switched (see “The Twente Photoacoustic Mammoscope: system overview and performance” Phys. Med. Biol. 50 (2005), pp. 2543-2557).
However, the conventional measurement apparatus for measuring spectral characteristics of the inside of a biological tissue has a following problem. The light propagating inside the biological tissue is affected by an anisotropy parameter g. The anisotropy parameter g has a value of approximately 0.9 in a biological tissue and mainly causes forward scattering.
On this occasion, energy of the light that is absorbed by an absorber in the biological tissue becomes larger in a position closer to the light incidence side due to an influence of the forward scattering.
As to a photoacoustic wave that is a photoacoustic signal generated from the absorber having a biased distribution of the energy of the absorbed light as described above, the signal generated from a boundary in the light incidence direction in which the energy of the absorbed light is large has the largest intensity.
In the structure described in U.S. Patent Application No. 2004/0127783, the transducer for detecting the photoacoustic signal is disposed on a plane different from that of the two illumination optical systems. In other words, the transducer is not disposed in the light incidence direction.
In a case where a photoacoustic signal generated from a spherical absorber is detected with the structure described above, a signal generated from a boundary at a position closest to the transducer is received first, and a signal generated from a boundary of the absorber at a position farthest from the transducer is received last.
From such a signal profile, the time for propagating in the spherical absorber and the sonic speed in the biological tissue are read, whereby the position and the size of the absorber can be calculated.
However, the position closest to the transducer and the position farthest from the transducer are in a direction different from the light incidence direction, and hence the propagation time cannot be detected with the signal of the largest intensity as described above.
In addition, according to Japanese Patent Application Laid-Open No. 2005-021380 and U.S. Pat. No. 5,840,023, the illumination optical system and the transducer for detecting the photoacoustic signal are disposed on the same plane, but the illumination optical system is disposed on only one plane.
With this structure, energy of light absorbed by the absorber becomes large on the light incidence side and becomes small on the opposite side thereto.
Therefore, the signal generated from the boundary at the position closest to the transducer can be detected with the largest intensity, but the signal generated from the boundary at the position farthest from the transducer cannot be detected with the largest intensity. Further, according to “The Twente Photoacoustic Mammoscope: system overview and performance” Phys. Med. Biol. 50 (2005), pp. 2543-2557, a pressed inspection object is illuminated on the both sides thereof one side by one side, but a time delay occurs because the illumination direction is switched.
With this structure, energies of light entering from both sides are not superimposed on each other, and thus the amount of light reaching the deep part of the tissue cannot be increased.
Therefore, intensity of a photoacoustic signal generated from the absorber in a deep part of the tissue becomes small. As described above, even if any one of the technologies described as conventional examples is used, there is a problem in detecting a position and a size of an absorber positioned in a deep part of a biological tissue with high accuracy and high contrast.